The present invention relates to a semiconductor device, a nonvolatile semiconductor memory, a system including a plurality of semiconductor devices or nonvolatile semiconductor memories, electric card including a semiconductor device or nonvolatile semiconductor memory, and an electric device with which this electric card can be used.
FIG. 12 shows the circuit configuration of a generally used nonvolatile semiconductor memory. This nonvolatile semiconductor memory includes a memory cell array MCA, address buffer ABF, column decoder CDC, row decoder RDC, sense amplifier S/A, fuse register FRG, I/O buffer IOBF, power-on reset circuit PORC, control circuit CT101, and voltage generation circuit VGC.
The memory cell array MCA has normal memory cell areas MC1 and MC2 for storing data, and a ROM fuse RF for storing data (to be referred to as fuse data hereinafter) required to be read out after the power source is turned on. Examples of the fuse data are fuse data for replacing defective portions in the memory cell areas MC1 and MC2 with other redundancy circuits, and trimming data for adjusting a timer and voltage generator.
Of input data to the address buffer ABF, a column address is input to and decoded by the column decoder CDC, a row address is input to and decoded by the row decoder RDC, and data write or read with respect to the memory cell array MCA is performed at the designated address. When data is to be read out, the data is output via the sense amplifier S/A and I/O buffer IOBF. When data is to be written, the data is supplied to the memory cell array MCA via the I/O buffer IOBF. Also, the fuse data stored in the ROM fuse RF is supplied to and held in the fuse register FRG via the sense amplifier S/A and column decoder CDC.
The voltage generation circuit VGC uses an externally supplied power source voltage VCC to generate various voltages such as a reference voltage Vref and program voltage Vpg.
The power-on reset circuit PORC outputs a low-level, power-on reset signal PWONRSTn before the power source voltage reaches a power-on detection level V2 after the power source is turned on. When the power source voltage reaches this power-on detection level V2, the power-on reset circuit PORC detects this and outputs a high-level, power-on reset signal PWONRSTn to the control circuit CT101.
While the power-on reset signal PWONRSTn is at low level, the control circuit CT101 initializes the address buffer ABF, fuse register FRG, column decoder CDC, sense amplifier S/A, row decoder RDC, and voltage generation circuit VGC by supplying a control signal indicating initialization to these components.
In addition, the above-mentioned fuse data stored in the ROM fuse RF must be read out and latched. If this fuse data is stored in a fuse circuit formed by a fuse which is blown by a laser, this fuse circuit is read. A read circuit for reading this fuse circuit is formed as a CMOS logic circuit. The level of a power source voltage with which this read circuit is activated is set to be equal to or higher than a voltage. Vlgc at which the CMOS logic circuit starts operating.
In the nonvolatile semiconductor memory as shown in FIG. 12, however, a specific area (the ROM fuse RF) in the memory cell array MCA can be allocated as a means for storing the fuse data.
In this case, the fuse data must be read out similar to normal data when the device is initialized. So, this data is readout from the ROM fuse RF (this operation will be referred to as ROM read hereinafter).
In this operation, as shown in FIG. 13, the power-on detection level V2 must be set higher than a minimum voltage V1 at which at least read operation is possible. Assume that, in a course during which the power source is turned on and the level rises, the power source voltage reaches the minimum operating voltage V1 at which read operation is possible at time T2, ROM read is started when the power source voltage reaches the power-on detection level V2 (V2>V1) at time T3, and this read is performed from time T3 to time T4. Accordingly, this ROM read is performed after initialization is complete. That is, the individual circuits are initialized after the power source is turned on and before the power source voltage reaches the power-on detection level V2. The power source voltage reaches the power-on detection level V2, and the power-on reset signal PWONRSTn changes from low level to high level, thereby completing the initialization. Then, ROM read is performed.
As described above, ROM read is desirably performed automatically when the power source is turned on (power ON), i.e., started immediately after initialization is complete after the power source is turned on. A signal for controlling this ROM read is generated by the control circuit CT101 which receives the high-level, power-on reset signal PWONRSTn output from the power-on reset signal PORC.
For example, a circuit shown in FIG. 14 is used in this power-on reset circuit PORC. The power source voltage VCC is divided by resistors R1 and R2, and a divided level N1 is supplied to the gate of a P-channel transistor PT1. This P-channel transistor PT1, a depression type transistor DT1, and a resistor R3 are connected between a power source voltage VCC terminal and ground terminal. The potential at the connection point of the drains of the transistors PT1 and DT1 is delayed by a delay circuit INC formed by an inverter array. This delayed potential is output as the power-on reset signal PWONRSTn.
The circuit configuration of the control circuit CT101 is as shown in FIG. 15. The power-on reset signal PWONRSTn input to this control circuit CT101 is supplied to a different control circuit OCT, a pulse generator PG11, and a ROM read controller RRC.
The pulse generator PG11 supplies, to a NAND gate NA11, a signal which is obtained by inverting the power-on reset signal PWONRSTn by an inverter IN11 and delaying this inverted signal by a delay circuit DL, and the power-on reset signal PWONRSTn, thereby generating a signal which is at low level only for the delay time. An inverter IN12 inverts this signal to apply an activation pulse ROMRDSTT to a set terminal Set.
The ROM read controller RRC is reset when the power-on reset signal PWONRSTn which is at low level after the power source is turned on is input to a reset terminal /Reset. When the activation pulse ROMRDSTT is input after that, the ROM read controller RRC generates a control signal for starting ROM read, and outputs this control signal to the individual circuits for performing ROM read.
Unfortunately, this conventional device has the following problems.
Assume that the power source is rapidly turned on. In this case, as shown in FIG. 16, the power source voltage VCC rises from time T0 and reaches the power-on detection level V2 at certain time. However, if this rise is too fast in accordance with the response of the power-on reset circuit PORC, the power source voltage is detected at time Tb as shown in FIG. 16, and the high-level, power-on reset signal PWONRSTn is output. Initialization is performed from time T0 to time Tb. As described previously, the ROM read operation is performed after time Tb.
At time Ta at which the initialization including ROM read has not been complete yet, the device status set in an R/B (Ready/Busy) pad indicates a ready state. Therefore, although the initialization has not been complete yet, the user may input a certain command by using an input program.
Also, to perfect the initialization when the power source is turned on, the specification generally recommends that a reset command be input after the power source is turned on. Accordingly, a reset command may be intentionally input.
If this reset command input at time Tb is ignored, reset may not be reliably performed because the user recognizes that the reset command is input and hence does not input the reset command again.
Furthermore, regardless of the reset command, the user may input a certain command, such as an initial value read command for reading out the initial value of software or the like or a status read command, before time Tb at which the initialization is complete. As in the above case, if this command input before the initialization is complete is ignored, an operation error may occur since the user recognizes that this command is input and hence does not input the command again.